Ichthyophthirius multifiliis is a unicellular freshwater fish parasite and the causative agent of the globally distributed white spot disease. The fitness of the parasite depends on available hosts and abiotic factors such as temperature, salinity and pH. With climatic change these abiotic factors may be altered, thereby influencing the health of the parasite. In this study, the tolerance towards different pH values (2–11) was investigated on a Nordic strain of the parasite by recording tomont survival, release of theronts, theront size and theront survival. Tomonts were able to survive and release theronts in pH 5–10, however the number of released theronts was significantly lower at high and low pH. Theronts produced at pH 8 and exposed to the different pH values survived at pH 4–10 for 1 h, which may be sufficient time for the parasite to locate and infect new hosts. The release of theronts was slower at pH 10, and the size of theronts developed at higher pH was significantly increased (up to 73.5 µm in length). In conclusion, our study showed that the free-living stages of I. multifiliis were capable of surviving at a pH from 5 to10, and that high pH had an effect on the morphology and release of the parasites.

鱼腥藻是一种单细胞淡水鱼寄生虫，是全球分布的白斑病的病原体。寄生虫的适应性取决于可用宿主和非生物因素，如温度、盐度和 pH 值。随着气候变化，这些非生物因素可能会改变，从而影响寄生虫的健康。在这项研究中，通过记录 tomont 存活率、theront 的释放、theront 大小和 theront 存活率，研究了北欧寄生虫对不同 pH 值（2-11）的耐受性。Tomonts 能够在 pH 值 5-10 下存活并释放 Theront，但是在高和低 pH 值下释放的 Theront 数量显着降低。在 pH 8 下产生并暴露于不同 pH 值的 Theront 在 pH 4-10 下存活 1 小时，这可能是寄生虫定位和感染新宿主的足够时间。在 pH 值为 10 时，theront 的释放较慢，在较高 pH 值下形成的 theront 的尺寸显着增加（长度可达 73.5 µm）。总之，我们的研究表明，自由生活阶段I. multifiliis能够在 5 到 10 的 pH 值下存活，高 pH 值对寄生虫的形态和释放有影响。